Theory Of Operation - HP 85020 A Operating And Service Manual

Model 85020A/B
Service
THEORY OF OPERATION
A1 Bridge Circuit
Figure 8-1. A1 Bridge Circuit Schematic Diagram
The 85020A/B Directional Bridae is a broadband device used to
measure return loss. The voltage across the balance arm {see
Figure 8-1) is proportional to the return loss of the device under
test. Placing a diode across the balance arm enables the
conversion of the microwave voltage to a 27 kHz sauare wave whose
amplitude is proportional to the return loss. The 1000-ohm
isolation resistors and the 1000-pF capacitors serve to isolate
the balanced detector from the Network Analyzer. This buffers the
output, reducing the imbalance across the center arm created by
the ground on one side of the output and the preamplifier
connection on the other.
Two parameters which define the quality of the bridge are
Directivity and Source Match.
Directivity is a measure of the residual error in the bridge, when
the test port is terminated in a perfect matched-impedance load,
the detector diode should read zero. Hov^ever, because of
non—symmetry due to resistor imbalances, component placement,
8-3